The present invention relates to a projection lens system for mixing images of a plurality of cathode ray tubes (CRTs) to enlarge and project the mixed images, and more particularly, to a projection lens system in which chromatic aberration is corrected.
Generally, using three monochrome CRTs each generating red, green and blue images as an image source, a video projector or projection television mixes the images and enlarges and projects the mixed color images forward onto a large screen. Since each of the three monochrome CRTs has narrow band of spectrum which is not of a strictly monochrome wavelength, they require a projection lens for correcting chromatic aberration, so as to realize a high-resolution color image.
A general lens in which the chromatic aberration is not corrected is designed on the basis of green wavelengths, and lens performances for red or blue wavelengths are approximately 30% less than that for green wavelength. Therefore, in order to realize high-resolution color images, a projection lens is needed wherein chromatic aberration is corrected for throughout the visible light band.
A projection apparatus for realizing a large image (greater than 100 inches) using three CRTs as an image source is roughly divided into two types: one having three CRTs and three lens units (see U.S. Pat. Nos. 4,963,007 and 4,900,139) and the other having three CRTs and one lens unit (see U.S. Pat. Nos. 4,764,806 and 4,842,394). Now, due to its easy adjustment of screen size, the latter apparatus is chiefly used.
In the triple-CRT single-lens projection apparatus as disclosed in the abovementioned U.S. patents, three CRTs are arranged in a "T" configuration. Each of the three CRTs has a concave image-generating surface having a small radius of curvature and generates a red, green or blue image. The images are mixed by means of two dichromatic mirrors which are disposed in a "X" configuration at the intersection of the light axes of the T-configured CRTs, and then the mixed images are enlarged and projected onto the forward screen by the projection lens. Thus, the projection lens projects a color image containing all the visible light wavelengths. However, conventionally, since the correction of chromatic aberration throughout the visible light band was not considered in designing such a projection lens, projecting color image having HDTV-level resolution on a large screen was impossible.